1. Field of the Invention
Radioactive isotopes are used in a variety of medical diagnostic techniques. With some of these techniques, a radioactive isotope is administered to the patient. Later, a study is made of the distribution and concentration of the isotope in the patient. This type of study is of benefit in diagnosing tumors and other ailments.
Prior to this invention and the gamma camera of the first referenced copending application, scintillation scanners were used in the usual techniques employed for the visualization of the spatial distribution of the radioactive material selectively absorbed in the tissues of the subject. In the usual scintillation scanning technique, a scintillation probe is moved along a series of parallel paths over the portion of the subject's anatomy that is being studied. Radiation from an administered radioactive isotope causes the probe to impart electrical impulses to a recording apparatus. This recording apparatus produces a graphic image of the spatial distribution of detected radiation. Typically, images are produced on paper by various types of recorders and on film by techniques known in the art such as those disclosed in U.S. Letters Pat. No. 3,159,744, entitled "Scintillation Scanner Photorecording Circuit" and U.S. Letters Pat. No. RE 26,014 entitled "Scintillation Scanner."
Because the probe is moved along these parallel paths, the time for conducting a complete scan is relatively long which results in a number of disadvantages. These include:
1. Relatively few patients may be examined with one apparatus;
2. The protracted period of time required results in discomfort of the patient; and
3. The image does not show the total distribution of radioactive substance in the subject matter at any one time.
Another type of proposed image system is sometimes referred to as a stationary scanner. This scanner uses a matrix of scintillators positioned over the area to be studied. Each scintillator within the matrix is associated with a specific location in the area to be studied. Light impulses generated by different scintillators in the matrix are read out by photomultiplier tubes. Impulses emitted by these tubes may then be used to reproduce the image formed by the radiation in a manner similar to that used in the scanning system described in the referenced patents. Although the matrix system is faster than scanning methods prior to it, the system is limited by the number of scintillators and photomultiplier tubes which can practically be used. Accordingly, resolution is poor.
A variation of the scanner using a matrix of scintillators is one with a scintillator positioned behind an apertured collimator. A plurality of photomultiplier tubes are positioned on the side of the scintillator opposite the collimator. Each time the scintillation appears on the scintillator it is viewed by a plurality of the photomultiplier tubes. The resulting impulses from the photomultiplier tubes are passed through a computer which determines the spatial location of the scintillation on the scintillator according to the relative strengths of the plurality of electrical pulses resulting from the scintillation. A graphic reproduction of the scintillations is then produced on the oscilloscope. Such devices are obviously complex and susceptible to some error due to variations such as from one multiplier tube to another. The spatial location of each scintillation is uncertain because of the poor statistical accuracy of each electrical signal. For these and other reasons such devices tend to fail to accurately demonstrate the true spatial distribution of radioactivity in the subject under study.
In the first referenced, copending application, another type of stationary scanner known as a gamma camera is described. There an image intensification tube is stimulated by photons emitted by the radioactive material administered to the patient. The image tube has an input phosphor which is stimulated by these photons and which emits light in response to them. The light emitted by the input phosphor stimulates an electroluminescent layer which emits electrons. These electrons are accelerated electronically against an output phosphor. The intensified image produced by the output phosphor then passes through a light amplification stage into a closed-circuit television system.
2. Description of the Prior Art
Image tubes in the prior art generally comprise an evacuated envelope with an input "sandwich" at one end of the tube, an output phosphor at the other end, and a means to electronically accelerate electrons emitted by the sandwich against the output phosphor.
This input sandwich typically includes a dishlike aluminum member which serves multiple purpose of: (1) filtering the input energy; (2) preventing external light from stimulating the input phosphor; (3) mechanically supporting the balance of the sandwich; and (4) a physical barrier against migration of molecules from the front of the input phosphor layer into the vacuum within the envelope.
The typical input phosphor is made up of many discrete particles of a phosphorescent material admixed in a suitable bonding and sealing vehicle such as an epoxy resin material. This admixture forms a slurry which is deposited in a thin layer on the inner surface of the aluminum dish to provide a phosphorescent layer of substantially uniform thickness with the phosphorescent material sealed off by the bonding agent or other barrier layer to prevent its migration into an electron-emissive layer or into the vacuum in the tube.
An electron-emissive layer is then deposited on this input phosphor and barrier layer. Typically, a microscopically thin film of metal will be interposed between the barrier layer and the electron-emissive layer to provide replenishment of electrons for the emissive layer. The metal layer is microscopically thin so that it does not materially inhibit the passage of light from the phosphor layer to the electron-emissive layer.
Electrons emitted by the emissive layer are accelerated by a suitable means such as electrostatic rings or a metal "jacket" which surrounds the acceleration path and which carries an electrical charge.
While image tubes of this type have been satisfactory for so-called bright fluoroscopic studies where an X-ray source provides the stimulating energy they severely limit the use of a gamma camera, especially when that gamma camera is utilized for conducting medical studies on human beings.
One reason image tubes of this type have limited the use of gamma cameras is that they have a very low so-called conversion efficiency. That is, a relatively low percentage of photons of energy entering the image tube are actually converted to light signals. One reason for this is that the typical input phosphor is relatively thin and much of the gamma energy passes through the phosphor without causing the emission of light.
The expedient of making the input phosphor thicker by prior known techniques has not been a satisfactory solution for gamma cameras. One reason it has not been a satisfactory solution is that each phosphorescent particle used to build up a fluorescent screen by this described technique has reflective surfaces. When enough particles are used to make a screen of a thickness which is efficient in converting the input energy to light, there are many reflective surfaces. Because there are so many surfaces, the stimulated light may reach the electron-emissive layer at a location which is, in a plane parallel to the plane of the input phosphor, a substantial distance from its source.
Obviously, this diversion of light will result in very poor resolution with the result that a produced graphic image will not provide an accurate indication of the true spatial distribution of the isotope in the subject under investigation.
While it has been known that halogens are efficient phosphors, they have not been accepted for use in image intensification tubes and the like because the halogens tend to disassociate relatively easily. This is especially true with iodides. A disassociated halogen atom tends to "poison" the vacuum by chemically interacting with the photo cathode; i.e., the electron-emissive layer, and even the output phosphor to cause deleterious effects and early tube failure.
Another problem has been how one can provide an input phosphor of the size requisite for a gamma camera or the like which produces accurate and dependable results with the relatively low stimulating energy available from a radioactive isotope. The above-described input phosphor layer is in reality a mosaic of discrete phosphorescent particles. Proposals have been made to develop a mosaic of larger pieces by adhering together a series of relative large crystals. According to these proposals, the crystals in the mosaic are positioned and adhered in a dishlike configuration which is generally parabolic in shape in order for the input "sandwich" to be focused relative to the electron accelerating field. These proposals also call for encapsulation of the mosaic to prevent poisoning of the image tube.
One major shortcoming of any mosaic is the conversion efficiency of each element of the mosaic is different from the others. The result is that a given light output of one crystal of the mosaic is not reflective of the same amount of incident energy as is the same light output from another crystal of the mosaic. This is true even where all crystals of the mosaic are cut from the same original crystal. The reason this is true is that the surfaces on each crystal cleaved from the large crystal is different from the surfaces on the other crystals, with different reflective indices. This results in a different light output in one crystal as compared to another for any given amount of incident energy.